1. Field of the Invention
The present invention relates to a control technology, and in particular to a gain control circuit and its gain control method.
2. The Prior Arts
In general, an analog type electronic gain control circuit is achieved through a Golbert cell. The basic conversion characteristic of Gilbert cell is Y=kX, wherein, Y is an output signal voltage, X is an input signal voltage, and k is a constant. From the above equation it is evident that, its amplification factor or voltage gain can be set at Y/X, and Y/X=k. Therefore, k is the voltage gain of output signal voltage Y relative to input signal voltage X.
In the following descriptions, refer to FIG. 1 & FIGS. 2(a) to 2(c) simultaneously. Compared with an analog type electronic gain control circuit, a digital type electronic gain control circuit has a wider dynamic operation range and more accurate control relative to an audio amplifier circuit 10. Wherein, FIG. 2(a) is a waveform diagram of output signals of a counter 18; FIG. 2(b) shows waveform diagrams of sound volume control voltage signal and saw-tooth wave signal generated by a saw-tooth wave signal generator 20, and the saw-tooth wave signal is generated based on a counter value output by the counter 18; and FIG. 2(c) is a waveform diagram of latch-up signals output by a comparator 22. The basic operation principle of a digital type electronic gain control circuit is that, a comparator 22 is used to compare a sound volume control voltage signal with a saw-tooth wave signal, when the voltage of the sound volume control voltage signal is equal to that of saw-tooth wave signal, the comparator 22 outputs a latch-up signal to a status register controller 24, so as to latch-up the contents of counter 18. The contents of the counter 18 are used to control the resistances of resistors 14 and 16 in an audio amplifier circuit 10, hereby determining the voltage gain of the amplifier circuit 10. However, under a specific voltage gain, in case that the voltage of an input signal of an amplifier circuit 10 is overly large, then the voltage of the signal output by the amplifier circuit 10 may exceeds a rated voltage, thus creating serious distortion. In other words, in this situation, the maximum voltage of output signal is restricted at a power supply voltage, and its minimum voltage of output signal is restricted at a ground voltage.
In view of the problems and shortcomings of the prior art, the present invention proposes a gain control circuit and its gain control method, so as to solve the problem of the prior art.